Microstructural and Mechanical Properties of Additive Friction Stir Deposited Aluminum 7050 Alloy

Technical Summary: Microstructural and Mechanical Properties of Additive Friction Stir Deposited Aluminum 7050 Alloy

Problem Statement
High-strength, precipitation-hardened 7xxx series aluminum alloys, specifically AA7050, are critical for aerospace airframes due to their specific strength and damage tolerance. However, conventional fusion-based additive manufacturing (AM) techniques struggle with these alloys, often resulting in solidification defects such as hot cracking, segregation, and porosity, as well as elemental volatility (Zn and Mg) that compromises hardening potential. While solid-state AM approaches like Additive Friction Stir Deposition (AFSD) offer a melt-free route to mitigate these issues, the specific relationships between AFSD processing parameters, microstructural evolution, and the resulting mechanical performance of AA7050 remain incompletely understood. Specifically, there is a gap in understanding how interlayer defects and bimodal microstructures affect the ductility and fracture mechanisms of as-deposited AA7050, and how post-processing treatments like Hot Isostatic Pressing (HIP) influence the trade-off between defect elimination and precipitate stability.

Methodology
The study utilized AA7050 feedstock to fabricate blocks via AFSD using a Bond Technologies GL7 instrument operating in force-controlled mode. The process involved a textured shoulder tool rotating at 450 rpm (pre-heat) and 250 rpm (deposition) with a traverse speed of 150 mm/min and a constant axial force of 14 kN. The resulting blocks achieved a density of 99.85%.

To evaluate the impact of post-processing, a portion of the as-deposited material was subjected to Hot Isostatic Pressing (HIP) at 470°C under 100 MPa argon pressure for 4 hours. The microstructural evolution was characterized using Electron Backscatter Diffraction (EBSD) to analyze grain size, orientation, and Kernel Average Misorientation (KAM), alongside Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) for phase identification. X-ray Diffraction (XRD) was employed to identify precipitate phases. Mechanical properties were assessed through room-temperature miniature tensile testing in the build direction, with fractography performed via SEM to correlate failure modes with microstructural features. Results were benchmarked against commercial wrought AA7050 (T7451 temper).

Key Contributions and Results

• Microstructural Architecture: The as-deposited AFSD AA7050 exhibited a distinct bimodal microstructure. The layer interiors contained coarse grains with a high fraction of low-angle grain boundaries (LAGBs), indicating dynamic recovery. In contrast, the reheated interlayers consisted of ultra-fine equiaxed grains with high-angle grain boundaries (HAGBs) and extremely high geometrically necessary dislocation (GND) densities, indicative of continuous dynamic recrystallization (DRX). This created a layered hierarchy not as uniformly refined as some previous friction-stir studies.
• As-Deposited Mechanical Performance: The as-deposited material retained significant strength (Ultimate Tensile Strength [UTS] of 404–468 MPa, or 80–92% of wrought AA7050) but exhibited a complete loss of ductility (0% uniform elongation) and failed without necking. Fractography revealed that this brittleness was not intrinsic to the matrix but was governed by large pores and lack-of-bond defects concentrated at interlayer boundaries. These defects acted as potent stress concentrators, initiating cracks at very low global strains.
• Effective Strength Analysis: By quantifying the pore area fraction in the gauge sections (ranging from 9% to 20%) and calculating the effective load-bearing area, the study demonstrated that the "effective" UTS of the as-deposited samples was approximately 505–509 MPa. This value is nearly identical to the wrought reference, confirming that the AA7050 matrix itself retained its intrinsic strength, and the observed mechanical degradation was purely a result of geometric defect reduction.
• Impact of HIP: HIP treatment successfully eliminated large interfacial pores and unbonded regions, restoring ductility to ~18.4%, comparable to wrought AA7050. However, this came at the cost of strength, with UTS dropping to a range of 316–324 MPa (average 320 ± 6 MPa). The HIP thermal cycle caused significant microstructural changes: the ultra-fine interlayer grains coarsened, and the strengthening precipitates (η' and η) underwent dissolution and coarsening into larger, equilibrium η-MgZn₂ rods. Additionally, coarse Fe-Cu rich intermetallics (Al₇Cu₂Fe) persisted, further depleting the matrix of strengthening solutes.
• Fracture Mechanism Transition: The fracture mode shifted from defect-controlled brittle failure in the as-deposited state to matrix-controlled ductile micro-void coalescence in the HIPed state. The HIPed samples exhibited a dense population of fine dimples, indicating that once large defects were removed, the material failed via the intrinsic ductile mechanisms of the alloy.

Significance and Claims
The paper claims to quantitatively demonstrate a strong trade-off between defect elimination and precipitate stability in AFSD-processed AA7050. The authors assert that while AFSD can produce high-strength AA7050 with near-wrought matrix properties, the as-deposited state is critically sensitive to interlayer porosity and bonding defects, which completely negate ductility.

The study concludes that achieving wrought-like strength-ductility combinations in AFSD AA7050 requires a dual approach:

1. Process Control: Optimizing AFSD parameters and toolpaths to minimize the formation of large pores and lack-of-bond regions during deposition, rather than relying solely on aggressive post-processing.
2. Thermal Management: Designing thermal treatments (including HIP and aging) that close residual porosity without causing excessive coarsening of the strengthening η' precipitates.

The authors emphasize that current AFSD designs for AA7050 must simultaneously control porosity, interlayer bonding, and precipitate stability to avoid the severe strength loss observed when relying on standard HIP cycles that over-age the material.